The present invention relates to echo cancellation techniques which are used in full-duplex data communication systems.
In the field of data communications, it is often advantageous for data traffic to be carried over a single communication channel in both directions simultaneously. Such simultaneous, two-way transmission is known as `full-duplex` operation. A dialed telephone connection within the Public Switched Telephone Network (PSTN) represents a typical medium over which full duplex data transmission, using modems, is both desirable and common, despite the fact that the network was designed for voice conversations.
One problem attendant to exploiting the PSTN for full-duplex data transmission involves the fact that a portion of the PSTN channel (from the telephone central office to each customer's site) consists of a single twisted pair or "loop", rather than two separate transmission media, one dedicated to each direction of transmission. In order to accommodate full-duplex operation, signal energies from both directions of transmission must be superimposed on this single loop simultaneously. On the other hand, within the equipment terminating the loops (i.e., within the modems and at the central office) it is necessary for the signal energies from each direction to be separated from each other. Separation of the two signals from the loop is accomplished by the use of so-called "hybrid balance networks" (or just "hybrids") within the terminating equipment at each end of the loop.
Ideally, the hybrids completely decouple the two superimposed signals within the loop, making it appear as though there were separate end-to-end transmit and receive loops, one for each direction of transmission. In practice, the hybrids do not achieve perfect separation, and some signal energy from each direction of transmission inevitably leaks through to the opposite direction path. Such cross-path leakage allows some of the modem's transmitted signal to appear at the input of its own receiver. This undesired leaked-through signal is often referred to as "echo". Typical hybrid leakage levels result in echo sufficient to seriously degrade a modem receiver's performance if not mitigated.
One way of avoiding the echo problem is to design full-duplex PSTN modems which transmit and receive using different frequency bands, effectively placing the echo energy outside the passband of the receiver. However, in order to maximize data rate versus performance, it is highly desirable to utilize the full channel bandwidth in both directions. Although the echo cannot simply be filtered out in this case, it can be reduced to acceptable levels through the use of an adaptive "echo canceller" within the modem. The echo canceller adaptively synthesizes a replica of the echo, which is then subtracted from the received signal, leaving a substantially echo-free signal. This echo-compensated signal is applied to the modem's data receiver, which typically includes an adaptive equalizer to mitigate the effects of linear impairments present on the channel.
Other techniques to facilitate full-bandwidth full-duplex operation are also possible, including the use of adaptive hybrid balance networks to reduce the leakage energy. However, the echo canceller technique has been so successful that is now is a standard component in most full-duplex high-speed PSTN modems.